It's not a line. Something can also be both aggressive and assertive. That they are different does not make them mutually exclusive.
One can also be awkward but not polite, polite but not awkward, and politely awkward. They're just different characteristics.
edit: I'm saying - where you draw the line is about things like whether it's competent, and whether it's "OK" within the contex. A polite "excuse me" is both competent and almost always "OK". A polite legal threat might not be "OK" (and it might not be competent either) so whether you judge it to be aggressive will depend on your point of view.